1. Field of the Invention
The present invention relates to a pulse-wave-propagation-velocity-related-information obtaining apparatus for obtaining information (e.g., a pulse-wave propagation time or a pulse-wave propagation velocity) that is related to a velocity at which a pulse wave propagates in a living subject.
2. Related Art Statement
Arteriosclerosis is known as a factor that influences pulse-wave-propagation-velocity-related information. This is utilized to evaluate a degree of arteriosclerosis of a living subject based on the pulse-wave-propagation-velocity-related information. A pulse-wave propagation time as a sort of pulse-wave-propagation-velocity-related information can be obtained by measuring a time difference between a time of detection of a heartbeat-synchronous signal at one portion of a living subject and a time of detection of the heartbeat-synchronous signal at another portion of the subject. A pulse-wave propagation velocity can be obtained by dividing the pulse-wave propagation time by a distance between the two portions of the subject. For example, an apparatus disclosed in Japanese patent document No. 9-122091 or its corresponding U.S. Pat. No. 5,743,856 detects, as a heartbeat-synchronous signal, an electrocardiographic signal (electrocardiogram) and a brachial pulse wave, and calculates, as a pulse-wave propagation time, a time difference between a time of detection of an R-wave of the electrocardiogram and a time of detection of a peak of the brachial pulse wave.
Since arteriosclerosis is directly related to hypertension, it is desirable that like blood pressure, arteriosclerosis be routinely measured at home. However, in the case where pulse-wave-propagation-velocity-related information is obtained by detecting a heartbeat-synchronous signal at two portions of a living subject, as described above, it is needed to employ and wear two heartbeat-synchronous-signal detecting devices, and yet it is considerably cumbersome to wear the two heartbeat-synchronous-signal detecting devices on the subject. Thus, it has been difficult to routinely measure pulse-wave-propagation-velocity-related information at home.
It is therefore an object of the present invention to provide a pulse-wave-propagation-velocity-related-information obtaining apparatus which can obtain pulse-wave-propagation-velocity-related information, with a single heartbeat-synchronous-signal detecting device (e.g., a pulse-wave sensor) worn on a living subject.
The Inventor has carried out extensive studies to achieve the above object, and has found that an observed pulse wave (i.e., an observed wave) is a composite wave containing an incident wave produced from the heart of the subject and a reflected wave that is produced from the heart and then reflected at a certain reflection point and that a time difference between the incident-wave component and the reflected-wave component, contained in the composite wave, indicates a time needed for the pulse wave to propagate over a distance equal to twice the distance between the observation point and the reflection point. The present invention has been developed based on those findings.
The above object has been achieved by the present invention. According to the present invention, there is provided an apparatus for obtaining information related to a velocity at which a pulse wave propagates in a living subject, the apparatus comprising (a) a pulse-wave sensor which is adapted to be worn on a body portion of the subject and detects the pulse wave from the body portion, (b) an incident-wave-peak determining means for determining a peak of an incident-wave component which is contained in the pulse wave detected by the pulse-wave sensor, (c) a reflected-wave-peak determining means for determining a peak of a reflected-wave component which is contained in the pulse wave detected by the pulse-wave sensor, and (d) an information obtaining means for obtaining the information based on a time difference between the peak of the incident-wave component determined by the incident-wave-peak determining means and the peak of the reflected-wave component determined by the reflected-wave-peak determining means.
According to the present invention, the incident-wave-peak determining means and the reflected-wave-peak determining means determine the respective peaks of the incident-wave and reflected-wave components of the pulse wave detected by the pulse-wave sensor, and the information obtaining means obtains the information based on a time difference between the peak of the incident-wave component and the peak of the reflected-wave component. Thus, the present apparatus can obtain, with the single pulse-wave sensor, the pulse-wave-propagation-velocity-related information.
According to a preferred feature of the present invention, the obtaining apparatus further comprises an arteriosclerosis-degree determining means for determining a degree of arteriosclerosis of the subject, based on the information obtained by the information obtaining means, according to a prescribed relationship between degree of arteriosclerosis and information related to velocity of propagation of pulse wave. Thus, the present apparatus directly evaluates a hardness of arteries of the subject as the degree of arteriosclerosis determined by the arteriosclerosis-degree determining means.